Electrolytic capacitors have a feature such that they have a large capacitance even in a small size, and they are widely used in a low frequency filter and a by-pass. The electrolytic capacitors generally have a structure such that an anode foil and a cathode foil are together spirally wound via a separator, and placed and sealed in a casing (see FIGS. 1 and 2). As the anode foil, a metal such as aluminum or tantalum, having an insulating oxide film formed thereon as a dielectric layer is generally used, and as the cathode foil, an etched aluminum foil is generally used. For preventing an occurrence of short-circuiting between the anode and the cathode, the separator disposed between the anode and the cathode is impregnated with an electrolyte, and it functions as a substantial cathode. Thus, the electrolyte is an important constituent which largely affects the properties of the electrolytic capacitor.
Among the properties of the electrolyte, an electrolytic conductivity directly affects the energy loss and impedance properties of the electrolytic capacitor, and therefore, vigorous studies are being made on the development of an electrolyte having a high electrolytic conductivity. For example, electrolytes comprising a quaternary ammonium salt (Japanese Prov. Patent Publication Nos. 145715/1987 and 145713/1987) or a quaternary amidinium salt (International Patent Publication No. WO95/15572 and Japanese Prov. Patent Publication No. 283379/1997) of phthalic acid or maleic acid dissolved in an aprotic solvent such as γ-butyrolactone, have been proposed. However, these electrolytes have unsatisfactory ionic mobility and unsatisfactory anodization of the anode aluminum, and therefore, they can be used only in capacitors at a rated voltage of 35 V or lower in general. Specifically, in these electrolytes, generally, only those having an electrolytic conductivity X as low as about 13 mS·cm−1 or less and a withstand voltage Y as low as about 100 V or less are obtained, and the electrolytes having an electrolytic conductivity X as relatively high as 13 mS·cm−1 have a withstand voltage as low as about 60 V, while the electrolytes having a withstand voltage Y as relatively high as 100 V have an electrolytic conductivity as low as about 8 mS·cm−1.
The electrolyte for an electrolytic capacitor is required to have higher electrolytic conductivity, more excellent thermal stability and more excellent voltage proof property, and it is needed to own all of these properties simultaneously. Further, the electrolytic capacitor is required to have lower impedance, more excellent thermal stability and more excellent voltage proof property, and it is needed to own all of these properties simultaneously. However, an electrolyte for an electrolytic capacitor and an electrolytic capacitor which meet such requirements have not yet been realized.